The metabolism of C apolipoproteins has been studied in normal subjects. The metabolism of the C apolipoproteins are of major importance since apoC-II is the cofactor for lipoprotein lipase, the enzyme involved in the catabolism of triglyceride rich lipoproteins (chylomicrons and VLDL). ApoC-II and apoC-III turnover studies were performed in 7 normal volunteers. VLDL, apoC-II, and apoC-III were radioiodinated, injected into the subjects and the decay of apoC-II and apoC-III from plasma and the plasma lipoprotein subclasses was followed with time to determine the parameters of their metabolism. The fractional catabolic rate of apoC-II was slightly shorter than apoC-III. The shape of the apoC-II and apoC-III decay curves were markedly different in VLDL when compared to HDL. The findings have been interpreted to indicate that apoC-II and apoC-III are metabolized independently and that the metabolism of these apolipoproteins is different in separate lipoprotein density classes.